What indicates a negative result in a Mantoux intradermal skin test?

In a Mantoux intradermal skin test, the primary indicator of a negative result is the absence of significant induration, generally considered to be less than 5 mm after 48 hours. The presence of a 3 mm induration specifically demonstrates that there is no meaningful response to the tuberculin antigen, therefore indicating a negative test result for tuberculosis exposure.

A 5 mm induration is typically on the threshold of what may be considered a potential positive result, particularly in certain high-risk groups, and redness alone is not a definitive measure of a positive or negative response as it does not indicate induration. A positive result for tuberculosis would also not reflect a negative result; rather, it would confirm exposure or infection. Thus, a 3 mm induration after 48 hours very clearly indicates a negative reaction in the context of the Mantoux test.